Switch grass fuel objects with high heat output and reduced air emissions designed for large-scale power generation

ABSTRACT

A Novel fuel object comprised of a proportion of switch grass and a proportion of wood fiber combined with a basically reacting compound. The fuel comprises fiber of the appropriate size and moisture content combined with an inorganic base. An appropriately sized fuel object is readily manufactured, provides high heat output, is consistent in fuel characteristics, and is sized and configured for use in power generation facilities. Based on fiber selection and processing, the fuel object may be used in a variety of current power generation technologies including stoker, fluidized bed, gasifier, cyclonic, direct-fired, and pulverized coal technologies, and results in significant reduction of air emissions (including sulfur dioxide, nitrogen oxides, hydrochloric acid, carbon monoxide, carbon dioxide, and mercury) compared to coal with no loss of boiler or furnace efficiency.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority to and the benefit as a continuationapplication of U.S. patent application entitled, “Switch Grass FuelObjects With High Heat Output And Reduced Air Emissions Designed ForLarge-Scale Power Generation”, Ser. No. 12/359,790 filed Jan. 26, 2009,the entire contents of which are incorporated herein by reference andrelied upon.

BACKGROUND

The invention relates to a high-energy, low-emission solid fuel madefrom natural renewable feedstocks.

In large part, energy generation in the United States and worldwide isbased on combustion of conventional non-renewable fossil fuels such ascoal, coal byproducts, petroleum-based oils and natural gas products.These energy sources provide a source of energy for power plants,industrial facilities, and institutions, however, they are not renewablein nature, have significant environmental impacts, are decreasing insupply, and increasing in cost. Continued use of these fossil fuelenergy sources results in cumulative environmental impacts includingincreased local and global concentrations of greenhouse gases, sulfurdioxide, nitrogen oxides, and mercury.

The decreasing supply and environmental impacts associated with fossilfuel use have led to consideration of potential for energy derived fromcombustion of natural products, typically cellulosic materials. Many ofthe natural sources comprising potential cellulosic fuels have notachieved commercial success in the past due to a variety of problemsincluding high moisture content, low fuel heating value, impurities inthe fuel, inconsistent fuel characteristics, transportation costs,difficulties in handling, and high processing costs. Further, suchmaterials often have combustion problems or compositions that result inthe formation of adverse emissions and substantial quantities of ash.

We have found, in the energy market segment, a substantial need for anew cellulosic based fuel containing substantially no conventional BTUsource from fossil fuels such as coal, petroleum, natural gas or othersuch non-renewable sources. This need relates to a fuel that has asubstantial heating value, is consistent in nature, is low in moisture,can be readily made at low cost, can be transported and handled at lowcost, can be used in existing solid fuel systems with little or nomodifications, has lower emissions than fossil fuels, and isspecifically adapted for use in modern power plant installations.

SUMMARY

We have now found a fuel source that provides a substantial heat outputsatisfactory for use in large-scale power generation, even in theabsence of coal, oil, gas or other conventional fossil fuels. A formedbriquette or object is comprised of a blend of cellulosic materialincluding switch grass and wood. As used herein, the term “cube,”“object,” “briquette,” “formed object,” or “fuel object” are roughlysynonymous and refer to a discrete particle of any size or shape thatcontains the natural cellulosic materials described herein. The majordimension of the fuel object is less than about 6 cm. The volume of thefuel object is about 10 to 100 cm³. “Conventional fossil fuel” refers tocoal products including bituminous coal, anthracite coal, peat, coke andcoking byproducts and to petroleum products such as oil, gas, naturalgas liquids and products derived from shale and tar sands.

We have found that moisture content and particle size of the switchgrass and wood fiber particles in the final fuel object are importantfor product formation, handling, and effective combustion.

We have found that the addition of an effective amount of a chemicalbase material reduces corrosive and acidic byproducts from thecombustion of the cellulosic materials and reduces emissions of sulfuroxides, nitrogen oxides, hydrogen chloride and other acidic materials.The processing and blend of materials provides a high energy outputwithout the addition of any fossil fuels such as coal, oil or naturalgas as found in prior art materials.

We have also found that the fuel object of the invention can be madewithout conventional binder materials such as that used in forming anumber of the prior art materials. Such binders, in the prior art, aretypically polymeric binders or are additional lignent or hemicellulosesmaterials.

We have found that the fuel object, based on sizing and specifications,may be used immediately in existing solid fuel energy facilities(including those employing stoker, fluidized bed, gasifier, cyclonic,direct-fired, and pulverized coal technologies) and operate with anefficiency and higher heating heat value similar to some coals with asignificant reduction in air emissions per million BTU of energy outputcompared to fossil fuels.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a flow chart of the process for forming the Fuel object ofthe invention.

DETAILED DESCRIPTION

The embodiment that is the subject of this application includes a fuelin the form of an object, briquette, cube or other such formed objectcomprised of switch grass, wood and an inorganic base. The inventioninvolves a fuel object processed, sized and configured for use inlarge-scale modern combustion energy generating systems. Composition,particle size, moisture content, and fuel object size and structure,result in the fuel object having a high energy value, low emissions uponcombustion, and highly efficient combustion, in a range of commerciallyavailable combustion technologies.

A fuel object can have a volume of about 10 to 100 cm³; does not need tobe symmetrical, but it is preferred that the fuel is in the form of anobject substantially symmetrical in shape such as cylinder, cube, solidparallel-piped or the like. A fuel object can be roughly a cylinder,rectangular prism or a cube that is 1 to 6 cm on a side. Typical densityof the fuel object is 30 to 40 lbs/ft³.

The fuel object typically comprises about 60 to 80 weight percent switchgrass and 20 to 40 weight percent wood. The switch grass component has aparticle size of about 80 to 25,000 microns with about 85 percent of theparticles greater than 1,000 microns. The wood fiber has a particle sizeof about 100 to 30,000 microns with about 90 percent of the particlesgreater than 1,000 microns.

The reduction size of the switch grass and wood fiber to theseproportions provides an input to the process that forms the fuel objectleading to a mechanically stable fuel object that can be manufactured,stored, transported and used in modern combustion installations as is oris easily comminuted to a small particle size depending on the nature ofthe combustion process.

Chemical bases that can be used in a fuel object of the inventioninclude typically alkali metal and alkaline earth metal bases. Suchbases can be made from sodium potassium, calcium, magnesium and othersuch metal species. The base can be used in the form of oxides,hydroxides, carbonates, bicarbonates, phosphates, and any otherinorganic anion that produces a basically reacting solution, a pHgreater than 7.5, when the base is mixed in at an amount of about 0.5 to10%. The important characteristic of the chemical base is that duringthe combustion process the chemical base can react with combustionbyproducts such as sulfur oxides, nitrogen oxides, chloride,hydrochloric acid and other acidic producing gaseous species neutralizedsuch species and substantially reduced the effects of corrosive actionon combustion equipment. Specifically, switch grass has higher chlorinecontent than wood and use of the base additive helps reducechlorine/chloride gases upon combustion of the fuel object. A preferredbase to neutralize acids in combustion is a bicarbonate or dolomite.

The fuel objects have a typical heating value of at least about 7,000BTU/lb. (about 3,500 cal.-gm⁻¹); about 7,300 BTU/lb (about 3,650cal.-gm⁻¹) and typically at least 8,000 BTU/lb (about 4000 cal.-gm⁻¹)all on a dry basis.

FIG. 1 is a diagram of the process for the manufacture of the fuelobject of the invention. The process 500 includes, as primary processstations, raw materials and fuels storage 501, raw materials dryer unit503, a cooling unit 504 and a cubing or object forming unit 505. Theseprimary stations take blended wood fiber and renewable fuel component,adjust particle size, reduce moisture to a preferred level, cool thematerial and then form the fuel object as needed. Once formed, the fuelis then stored or transferred to an industrial combustion or burn unit(not shown).

The raw materials used in making the object of the invention isdelivered to and stored in raw material delivery and storage unit 501.That material is then transferred to a pretreatment screen 511 for thepurpose of separating fines from useful material. Useful material isthen transferred to a pretreatment hammer mill which adjusts the fibersize to appropriate fiber dimensions for final object formation. Finematerials from both the prescreening, pretreatment and from thehammermill are transferred to the colt exhaust bag house 515 through theconduit A-A. The sized fuel is transferred to a feed bin 512 fortemporary storage. The sized fuel source is then transferred using adryer and feed conveyor 513 to the dryer drum 503 for drying purposes.The output from dryer 503, having moisture content of about 0.1 to 14wt. % moisture, is then directed to drop out chamber 514(a) thatseparates fines from the appropriately sized materials in a dried form.Heat for the dryer 503 is generated by Burner 506 and blending chamber507. Fuel for the burner 506 is stored in bin 510. Fuel is transferredto the burner 506 through transfer line B-B from bin 510. Heat andrecycle gas stream from dryer 514 b is sent conduit 508 through chamber507 to the dryer 503. Heat is recycled to the burner 506 throughconveyor 509.

The fines are directed to cyclone dryer 514(b) while the appropriatelysized materials are directed to cooling drum in feed conveyor 517.Exhaust from the cyclone material is directed to recycle fan 522 whichdirects the exhaust either to ambient air or to the recycle throughblending chamber 507. The cold exhaust bag house 515 takes fines fromthe screen and hammermill station 511 and from the cubing stations 505.Those bag house fines are collected on the particulate conveyor 516which are combined from the output from the cyclone dryer 514(b) and aredirected to the pulling drum and feed conveyor 517. Cooling drum 504cools the particulate material from the infeedment conveyor 517 and thenconveys that material on conveyor 519 to the fuel object formationstations 505. Optionally, if the cooling drum is not required forprocessing the fiber, the fiber before entry into the cooling drum inconveyor 517 can be directed to a dryer bypass bulk conveyor 518 andthen to an optional dryer cooling drum bypass feed conveyor 523 thatdirects the fiber to the object formation stations 505. Fines fromscreen 511, through airlock 521, the object forming units 505 and fromdrum 504 or conveyor 517 are directed to baghouse 515. Air is vented toambient from baghouse 515 from vent 520. The input material placed indelivery and storage unit 501 is typically preblended with theappropriate amount of wood fiber and renewable fuel source. Once formed,the fuel objects of the invention can be stored in product stock pile502 and then transferred to a combustion unit (not shown) for energygeneration).

Reduced Air Emissions

The composition of the fuel and base additive are designed to minimizeair emissions from combustion of the fuel object. The fuel objectscontain less than 0.5 percent sulfur by weight percent and thereforeemit approximately 95 percent less sulfur dioxide emissions thanderivation of a similar amount of energy from coal. Because wood andswitch grass are biogenic in nature, combustion of the fuel object isconsidered carbon neutral under carbon registries and trading programsin place in the United States today and therefore results in a 100percent reduction in creditable greenhouse gas emissions than derivationof a similar amount of energy from coal. Nitrogen content of the switchgrass and wood and addition of the inorganic base additive also resultin an approximately 40 percent reduction of nitrogen oxide emissionsthan derivation of a similar amount of energy from coal.

Object and Particle Size

The size and density of the fuel object is significant in that it allowsfor ease of handling, transportation, storage and conveyance in mostpower generation facilities. Object size also is important in that itallows the fuel object to bum on the grate of stoker-type combustionunits and not combust prematurely.

Particle size within the fuel object also is significant in bothmanufacturing a fuel object that maintains its integrity though shippingand handling and that burns efficiently. Efficient combustion reducesemissions of nitrogen oxides and carbon monoxide and leaves minimalresidue, such as ash, which would have to be disposed in a waste site.Sizing of the fuel particles also is critical to allow a fuel object tobreak into discrete particles in certain applications such as pulverizedcoal-type units.

Combustion Efficiency

The blend, composition, moisture and size of the fuel object allowefficient operation in existing power generation facilities. Weanticipate no loss of boiler or furnace efficiency when using the fuelobject when compared to use of coal.

Switch Grass

The term “switch grass” refers to a summer perennial grass native toNorth America with the technical name of Panicum virgatum. Switch grassis a natural component of the tall-grass prairie species that coversmost of the great plains but is also found in prairie soils of Alabamaand Mississippi. Switch grass is naturally resistant to many pests andplant diseases and is capable of producing high yields with very lowapplication of moisture and fertilizer. Switch grass can also betolerant of poor soils, flooding, and drought.

There are two main varieties of switch grass upland and low land types.Upland types can grow to 6 feet tall and are adapted to well drainedsoils. Low land types grow up to 12 feet tall and are typically found onheavy soils and bottomlands. A number of varieties have been found forforage sources. As such, switch grass is a useful source of relativelylow moisture cellulosic material that can be produced in large volumesfor energy production.

Wood Fiber

The term “wood fiber” refers to a product derived from some part of atree as that term is commonly used in the art. A number of directproducts and byproducts can be derived by taking trees or portions oftrees and reducing their particle size. The term “wood fiber” may referto materials derived from fruit, leaves, sap, bark and other such treebyproducts. Wood fiber is typically derived from either the woody partof the tree within the bark and typically refers to either wood-likecomponents of tree trunks, tree limbs and tree roots. Wood fiber istypically primarily cellulosic in nature but is known to be derived fromwood cells that typically comprise a substantial proportion ofcellulosic materials in combination with lignin and hemicellulosicmaterials in a fibrous woody cell structure. Wood fiber can be derivedfrom a number of tree sources including both hard and soft woods. Suchwood fiber materials can be derived from the processing of trees intosized lumber, the byproduct of clearing and shredding trees, thebyproducts derived from any process that begins with a wood containingplant part leading to the formation of a substantially cellulosic woodfiber material.

General Method of Manufacturing: a Fuel Object

The process for manufacturing fuel objects described herein starts bygrinding cellulosic material. The cellulosic material can be ground byfeeding a pulverizer or grinder to reduce the cellulosic material to apredetermined size. The switch grass component is ground to a size ofabout 80 to 25,000 microns with about 85 percent of the particlesgreater than 1,000 microns. The wood fiber is ground to a particle sizeof about 100 to 30,000 microns with about 90 percent of the particlesgreater than 1,000 microns).

After grinding, cellulosic materials a may be fed through a dryer. Adryer ensures that moisture content of the cellulosic material is atless than about 14 wt. % and often less than 10 wt. %. Preferably,moisture of a finished fuel object should be between 7 wt % to 14 wt. %.Moisture content of the cellulosic material is significant to theintegrity of a fuel object since moisture content of cellulosic materialassists in bonding all of the materials in the composition prior to andfollowing the pelletizing of the composition. However, an increase inthe moisture of the cellulosic material beyond a disclosed limit wouldjeopardize the characteristics of the fuel object and its ability towithstand being transported. It is important that a fuel object maintainits integrity prior to precombustion processing or burning A fuel objectshould be rigid enough to be handled mechanically without crumbling.Achieving proper and desired moisture content for a fuel object iscritical to achieve a desired heat output and in maintaining the abilityto transport fuel objects without harming its integrity, shape, orcomposition.

After the cellulosic material has been sufficiently dried to desiredmoisture content, the cellulosic material can be fed through a secondarypulverizer as necessary. A secondary pulverizer can be the finalgrinding process for the cellulosic materials.

The base additive may be added once organic components have beenappropriately sized and conditioned. Components of a fuel object may befurther blended together by means of a blender, drum or other mechanicalequipment.

A densification process can create a final composition of materials.Densification allows materials to be mixed and blended in a controlledmanner with other particles comprising a fuel object. After thecomponents of a fuel object have been sufficiently blended, thecomponents/materials are processed and forced through a shape formingdie. Such equipment could include commercially available machines suchas those produced by Warren and Baerg. Shaping equipment forces ablended composition through a die of a forming machine, thereby creatinga fuel object. Faceplate temperature of the extrusion equipmenttypically is between 165° F. and 185° F. A fuel object exits a shapingdie at a temperature of about 110° F. and not greater than 145° F. Whena fuel object exits the shaping die, there can be a slight coating onthe external surface of the object. This coating can comprise lignin,which is a naturally occurring substance of the cellulosic material.Shaped objects are then transferred to the finished productconveyor/cooler.

Following formation, fuel objects are cooled down by a cooling meansincluding, but not limited to, an air cooler, an air conditioner, orliquid nitrogen. The cooling process causes the objects to harden intothe shape created by the forming equipment and allows components of theobject to maintain their integrity. In an embodiment, fuel objects areplaced through a shaker screen after sufficiently cooling and hardening.This process separates fine and discrete particles of the composition.The discharge for the fine particles can be separated from a fuel objectand are again recycled or forced through a shaping die. This processminimizes the potential for waste generated by any excess particles thatcomprise a fuel object. The final object maintains its structure andprovides an ease of handling and a highly consistent product for goodcombustion at the end user power facility.

Use Integration at Power Facilities

The fuel object is designed for immediate use in existing solid fuelfired systems. This may include facilities that produce heat or steamfor cooling, heating, or electrical generation or direct induration ordrying of a product. Such facilities may include power plants,industrial furnaces and boilers and steam and power generationfacilities at large institutions such as universities and hospitals. Theformed, shaped and densified fuel is ideal for transport by truck, rail,and conveyer and storage in bunkers and silos that are designed fortransport and storage of coal. In most instances the objects may beunloaded, stored, and transported at a facility by the existingmechanisms that transport coal without physical modification.

The fuel object is ideal for use in stoker fired systems where fuel isdischarged on a large grate and burns on the grate over a period oftime. The fuel object also works in systems where fuel is pulverizedprior to entry of the combustion chamber and then combusted insuspension such as pulverized coal, cyclonic combustion, anddirect-fired units. Because individual particles in the fuel object arereduced in size prior to cubing and dried to a low moisture content,they combust efficiently and with low emissions when fed through suchsuspension-based systems and do not result in slagging or increased ashor sparklers from unburned fuel. Because the fuels have such a highheating value, burn efficiently, and reduce emissions, facilities thatuse the fuel object may reduce emissions without capital expenditure onemission controls and maintain unit efficiencies.

Example Switch Grass Fuel Object

A fuel object derived from switch grass and wood was produced and wassubstantially free of coal. The switch grass fuel object wassubstantially cylindrical in shape with a length of 5.4 cm and adiameter of 2.6 cm.

Volume 14.04 cm    Moisture content 8.27 wt. % Amount of particulatederived from 63.2 wt. % switch grass- fiber size 80 to 25,000 micronsAmount of particulate derived from 34.4 wt. % wood fiber- fiber size 100to 30,000 microns Sodium bicarbonate (inorganic base)  2.4 wt. %

In testing, this switch grass fuel object provided about 7,386 BTU/pound(lb.).

Although the invention has been disclosed in the context of certainembodiments and examples, it will be understood by those skilled in theart that the invention extends beyond the specifically disclosedembodiments to other alternative embodiments and/or uses and obviousmodifications and equivalents thereof. Accordingly, the invention is notintended to be limited by the specific disclosures of preferredembodiments herein, but instead by reference to claims attached hereto.Reference to a single element in the claims is intended not exclude oneor more of the same element. The above specification, examples and dataprovide a complete description of the manufacture and use of thecomposition of the invention. The invention resides in the claimshereinafter appended.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A fuel object having maximum dimension of about 6 cm, a volume ofabout 10 to 100 cm³, the fuel object comprising: (i) about 60 to 80 wt.% of a particulate derived from switch grass having a particle size ofabout 80 to 25,000 microns; (ii) about 20 to 40 wt. % of a wood fiberhaving a particle size of about 100 to 30,000 microns; (iii) about 1-10wt % of an inorganic base; and (iv) less than about 14% of water,wherein the fuel object is substantially free of coal and polymericbinder, and the fuel and provides at least 7,000 BTU-1b.⁻¹ (3500cal.-gm⁻¹) upon combustion.
 2. The fuel object of claim 1 wherein thefuel object comprises a cubic or cylindrical unit with a side dimensionof 2 to 6 cm.
 3. The fuel object of claim 1 wherein the inorganic baseis selected from the group consisting of sodium bicarbonate, dolomite,and mixtures thereof.
 4. The fuel object of claim 1 wherein the BTUsource of the fuel object consists essentially of: (i) about 60 to 70wt. % of switch grass; (ii) about 30 to 40 wt. % of wood fiber; (iii)about 2 to 3 wt. % of an inorganic base; and (iv) about 8 to 14 wt. % ofwater.
 5. The fuel object of claim 1 wherein the density of the fuelobject is 30 to 40 lbs/ft³ (0.5 to 0.65 gm-cm⁻³).
 6. The fuel object ofclaim 8, wherein upon combustion, the fuel object produces a firstamount of energy and emits about 95% less SO₂ compared to combustion ofan amount of coal that provides substantially the same amount of energyas the first amount of energy.
 7. The fuel object of claim 10, whereinupon combustion, the fuel object produces a first amount of energy andemits about 40% less NO_(x) compared to combustion of an amount of coalthat provides substantially the same amount of energy as the firstamount of energy.
 8. The fuel object of claim 1, wherein uponcombustion, the fuel object produces a first amount of energy and emitsabout 95% less SO₂ and about 40% less NO_(x) compared to combustion ofan amount of coal that provides substantially the same amount of energyas the first amount of energy.
 9. A fuel object comprising: (i) about 60to 80 wt. % of a particulate derived from switch grass having a particlesize of about 80 to 25,000 microns; (ii) about 20 to 40 wt. % of a woodfiber having a particle size of about 100 to 30,000 microns; (iii) about1 to 10 wt. % of sodium bicarbonate; and (iv) less than about 14% ofwater; wherein the fuel object is substantially free of coal andconventional binder material, and, upon combustion, produces a firstamount of energy and emits about 95% less SO₂ and about 40% less NO_(x)compared to combustion of an amount of coal that provides substantiallythe same amount of energy as the first amount of energy.